This invention relates to an optical disc device, and in particular, relates to an optical disc device that can optimally adjust the amplitude of a high frequency signal to be superimposed on a drive signal of a laser beam source to reduce laser noise.
An optical disc device shines a laser beam emitted from a laser beam source onto an optical disc to play data recorded on the optical disc with laser light reflected on the surface of the optical disc. Because of the structure of the optical disc device, a part of the return light from the optical disc enters the laser beam source. The return light may remarkably increase noise in the laser beam. Hence, the optical disc device superimposes a high frequency signal onto a drive signal of the laser beam source so that the laser beam source emits an intermittent beam modulated with the high frequency signal, not a continuous beam, and the timing of the emission of laser beam is staggered from the timing of the entrance of return light to suppress variation of the intensity of the laser beam (for example, refer to JP2008-112578A and JP2009-110602A).
As described above, superimposition of high frequency in playing an optical disc provides stable playing performance. However, when the amplitude of the high frequency signal to be superimposed on the laser beam source is smaller and the laser beam is less modulated, the laser noise might not be sufficiently reduced even though a high frequency is superimposed. For example, variation of characteristics of laser diodes or variation of characteristics of optical systems may cause insufficient modulation. If the laser noise is not sufficiently reduced by these causes, data recorded on the optical disc may not be able to be played.
On the other hand, when an optical disc is played with a high frequency signal having excessive amplitude superimposed, the data recorded on the optical disc might be destroyed. Therefore, it has been desired that the amplitude of the high frequency signal to be superimposed be smaller to suppress the peak power of the laser beam.
For this reason, the amplitude of the high frequency signal is required to be adjusted to achieve an optimum modulation.
Conventionally, a method of determining the amount of high frequency signal to be superimposed based on the error rate of data read from an optical disc has been proposed. To measure the error rate, it is disadvantageously necessary to play the data from the optical disc (namely, to perform processes up to decoding). Before adjusting the amount of high frequency signal to be superimposed, adjustments of tracking and others are required, taking much time until determination of the amount of high frequency signal to be superimposed.
In addition, it is necessary to play data in a certain zone continuously for calculating the error rate; it took time to read the data for calculation of the error rate. Accordingly, a method for quick determination of the amount of high frequency signal to be superimposed has been desired.
In the meanwhile, a method of determining the amount of high frequency signal to be superimposed based on the amount of laser noise has been proposed. To measure the amount of laser noise, however, a dedicated detector which can detect a high frequency signal; the configuration of the optical disc device becomes complicated to increase costs.
An object of this invention is quick determination of the optimum amplitude of high frequency signal for the loaded disc with a simple configuration to achieve stable playing performance.